1. Field of the Invention
The invention relates to a method and arrangement for optimizing an amplitude-modulated optical signal.
2. Description of the Related Art
Digital signals are frequently transmitted in optical networks with the aid of amplitude modulation (ASK). A carrier wave is transmitted in the case of one logic state, and no signal/carrier wave is transmitted during the other logic state. Early in the modulation (on-off), a “chirp” is produced which changes the wavelength and amplitude of the output signal. The transient component of the chirp causes large variations in the edge regions, and a sharp increase or decrease in the wavelength; the switch-on edge is of particular importance since the changes occur in the case of a full signal level. The other adiabatic component of the chirp can be kept small by a suitable design of the modulator.
During transmission of a pulse in a waveguide (glass fiber), self-phase modulation of the carrier occurs (this being a further form of the chirp) in which the wavelength changes likewise, particularly in the leading edge region and trailing edge region of the pulse. Also, amplitude distortions can occur.
The two wavelength distortions, including the transient component of the chirp induced by switching on, and the self-phase modulation result in pulse distortion of the baseband signal, contribute (particularly in the case of transmission systems with high bit rates) to limiting the data rate and the transmission range.
An attempt is usually made to minimize the chirp-induced disturbing influences by setting operating points of Mach-Zehnder modulators or integrated electro-absorption modulators in a test bay. However, resettings must be undertaken when changes occur to the operating parameters.
British patent document GB 2 308 675 A discloses an arrangement and a method for driving an optical modulator and describes the setting of a chirp parameter where monitoring the modulated signal is performed at the receiving end in order to set the chirp parameters via a back channel for pulse compression.
British patent document GB 2 316 821 A describes an optical time-division multiplex system which compensates the chromatic dispersion of the transmission path by means of controlled chirping of the transmitted signal. Monitoring of the modulated signal is not provided.
Earlier European patent application EP 0 971 493 A1 likewise describes a method for compensating dispersion and nonlinearities in optical communication systems. In this system, however, it is, for example, the error rate which is measured and the transmission level which is controlled as parameters. Both measures do not appear to be expedient in modern optical systems.